Chromic Anhydride

CrO 3 . Syn. Chromic acid, Anhydrous chromic acid, Chromic trioxide . Prep. By conducting gaseous fluoride of chromium into a silver or platinum ve el, the sides of which are just moistened with water, and the aperture covered with a piece of moist paper, the anhydride will be deposited under the form of red, acicular crystals, which will nearly fill the ve el. When the proce is skilfully conducted, the product is of exquisite beauty and chemically pure. The fluoride referred to above is obtained from fluor spar, 3 parts; chromate of lead, 4 parts; fuming (or the strongest) sulphuric acid, 5 parts; mixed cautiously in a silver or leaden retort. A red-coloured gas is evolved, which acts rapidly on gla , forming fluosilicic acid gas, and upon water forming hydrofluoric acid and chromic anhydride. The moisture of the atmosphere is sufficient to effect the decomposition last referred to; the former substance escaping as gas, and the latter being deposited in small crystals. It is also prepared nearly pure by adding a cold saturated solution of pota ium bichromate to once and a half its bulk of pure strong sulphuric acid. As the liquor cools, the anhydrous chromic acid is deposited under the form of brilliant crimson-red prisms; the mother-liquor is then poured off, and the crystals, placed between two tiles of gla or porcelain, are submitted to strong pre ure for some time, under a bell-gla or jar, when the anhydride will be found sufficiently dry. It may be deprived of a little adhering moisture by placing it over sulphuric acid for a short time in vacuo . Commercially, it is prepared by one of the two following proce es:— To a saturated solution of chromate of pota ium, 100 parts, add oil of vitriol (sp. gr. 1·845), 49 parts; and let the whole cool. This is the common proce . The product contains sulphate of pota ium, but this does not much interfere with its value as a bleaching agent. From chromate of barium, decomposed by concentrated nitric acid. The anhydrous chromic acid is separated from the nitrate of barium by decantation, or, which is still better, by filtration through gla or asbestos. It is then evaporated to dryne , when the nitric acid is volatilised, and pure chromic anhydride left behind. The volatilised nitric acid may be condensed, and again used for the same purpose. The only precautions nece ary to ensure the purity of the anhydrous chromic acid prepared by this plan are—to use a sufficient quantity of nitric acid and to take care that the nitric acid is sufficiently concentrated and pure. Prop., . Forms ruby-red anhydrous prisms, very soluble in water, with formation of true chromic acid, and extensively manufactured for the purpose of oxidising and bleaching substances.